Non-oriented electrical steel sheet is used for rotor of rotating machine, for example. In general, centrifugal force exerted on the rotor is in proportion to the radius of rotation, and in proportion to the square of the rotational speed. Accordingly, a very large stress is loaded on the rotor of the high-speed rotating machine. The non-oriented electrical steel sheet for rotor is, therefore, preferably given large tensile strength. In other words, the non-oriented electrical steel sheet for rotor is preferably a high tensile strength steel. As described in the above, the non-oriented electrical steel sheet for rotor is required to have high tensile strength.
On the other hand, it is important for the non-oriented electrical steel sheet, used for iron core not only for the rotor of rotating machine, to have a low iron loss. In particular for the non-oriented electrical steel sheet for the rotor of high-speed rotating machine, it is important for high-frequency iron loss to be low. As described herein, the non-oriented electrical steel sheet for rotor is also required to have a low level of high-frequency iron loss. In other words, the steel is required to ensure high efficiency, when the rotating machine is operated at high frequencies.
High tensile strength and low high-frequency iron loss are, however, contradictory issues, which may be satisfied at the same time only with great difficulty.
While there have been techniques ever proposed aiming at satisfying the both at the same time, no technique capable of readily manufacturing such steel has been known. For example, a technique of obtaining a high-Si-content hot rolled steel sheet, followed by temperature control in various ways, has been propped. However, the technique suffers from difficulty in cold rolling, due to the high Si content. Moreover, the technique needs various temperature controls for enabling the cold rolling, and the controls are highly specialized, so that time, labor and costs consumed therefor are pushed up.